1. Field of the Invention
The present invention relates to an aqueous ink, and also relates to a method for ink jet recording, an ink cartridge, a recording unit, and an ink jet recording apparatus.
2. Related Background Art
As a colorant for print ink, pigments having excellent fastness such as water resistance and light resistance have widely been used. Also in ink jet recording ink, use of pigment as a coloring material has been actively studied to improve the fastness of the ink jet print. In order to use a pigment as a coloring material for aqueous ink jet ink, it is important to stably disperse the pigment in an aqueous medium. A dispersant, for example, a resin dispersant, is generally used in order to uniformly disperse the pigment in the aqueous medium. Such a resin dispersant generally comprises a water-soluble resin having a hydrophilic group for stable dispersion of the pigment in the aqueous medium and a hydrophobic portion to physically adsorb onto the hydrophobic surface of the pigment. The quality of images obtained with such pigment ink, for example, rubbing resistance and marker resistance, is relatively good due to the presence of the resin dispersant in the ink.
On the other hand, in order to produce high quality ink jet images stably, it is important to discharge stable ink droplets from fine nozzles of an ink jet recording head. Specifically, it is important for stable ink jet recording that ink does not solidify at the orifice of the ink jet recording head with drying of the orifice. At this point, the above pigment ink jet ink containing a resin dispersant for pigment dispersion has a problem. Since the above-described ink contains a substantial amount of free resin dispersant not adsorbed to the surface of pigment but dissolved in the ink, attachment of such dispersant to the orifice may cause nozzle clogging.
To address the above-described problem, methods for improving reliability of the pigment ink have been proposed. For example, Japanese Patent Application Laid-Open No. 8-3498 or Japanese Patent Application Laid-Open No. 10-195360 discloses an ink using as a coloring material a pigment that can disperse stably in an aqueous ink without using a dispersant (self-dispersing pigment) due to the water soluble groups introduced onto the pigment surface, specifically, of carbon black particles.
However, when printing is carried out with an aqueous ink containing the above-described self-dispersing pigment on a recording medium, particularly, on a plain paper, sometimes the rubbing resistance of the printed image is not sufficient, that is, strong rubbing of the print surface may cause smearing even after sufficient drying of the ink. As described above, dispensing with a resin dispersant in ink may impair certain qualities of an ink jet recording, although it effectively serves to further improve reliability of ink jet recording.
Japanese Patent Application Laid-Open No. 9-272831 discloses an ink-jet ink containing a self-dispersing pigment to which a resin is chemically bonded on the surface, and an ink jet recording process using the ink. It also discloses that the self-dispersing pigment can be obtained by heating a pigment having functional groups and a polymer consisting of segment (A) and segment (B) where segment A as reactive groups reactive with the functional groups on the surface of the pigment and segment (B) has substantially no such reactive groups and has higher affinity for the liquid medium than segment (A) does.
The above-described invention, however, has a problem that the polymer and pigment suitable for use are limited because it is essential for them to have specific functional groups reactive with each other. In addition, according to the study of the inventors of the present invention, it will be difficult in the manufacturing process to impart the ionic property to segment B where groups reactive with the pigment surface are not present. As a result, it is difficult to obtain pigments having sufficient dispersibility in an aqueous medium.
Japanese Patent Application Laid-Open No. 2000-53902 discloses an ink composition for ink jet printing which includes a macromolecule chromophore having at least one functional group for solubility in water and a polymer attached thereto via at least one covalent bond, wherein the polymer is attached to the macrochromophore by nucleophilic substitution or acylation. However, the above invention has problems that usable polymers are limited because they must have an amine or hydroxyl terminal within the polymer; and although rubbing resistance is improved due to dispersion of the functional groups of the terminal modified homopolymer, the marker resistance is insufficient since the polymer on the surface of the pigment is hydrophilic, and is dissolved into water or water-soluble organic solvent of the marker.
Japanese Patent Application Laid-Open No. 2000-95987 also discloses an ink composition for ink jet printing which comprises a macromolecule chromophore (pigment particles) having at least one functional group to give the chromophore solubility in water and at least one polymer attached thereto via a covalent bond, where the polymer is vinyl polymer. To prepare such a composition, radical polymerization of the monomer is conducted in the presence of an initiator and a modified pigment in the step of adding a polymer chain to the surface of the pigment via a covalent bond. According to the study of the present inventors, however, it is difficult to control the molecular size of the polymer in radical polymerization, and it is also difficult to control the acid value or amine value that is an indicator of solubility of the water-soluble polymer in water. In addition, it is not certain that all growing polymers are infallibly bonded to the surface of the pigment via a covalent bond, i.e., terminal radicals of the growing polymers may not necessarily bind to the surface of the pigment to terminate polymerization, but the polymerization reaction may be terminated by binding between growing polymers while part of the growing polymers bind to the surface of the pigment. Therefore, there is a problem in manufacturing the ink composition that it is very difficult to efficiently bind the polymer to the surface of the pigment.
WO 01/51566A1 describes a resin-bonding type self-dispersing pigment using a plurality of molecules. This resin-bonding type self-dispersing pigment allows dispersion of pigment having ionicity. However, the present inventors have found that such use of such a resin-bonding type self-dispersing pigment can secure reliability of the ink with the recording head, but the rubbing resistance or marker resistance of the printed images still has room for improvement.